Polyester materials are widely used as extrusion and injection molding resins for applications such as fibers, films, automotive arts, and food and beverage containers. Commonly used polyesters include poly(ethylene terephthalate) (PET), poly(1,4-butylene terephthalate) (PBT), poly(1,4-cyclohexylene-dimethylene terephthalate) (PCT), and poly(ethylene 2,6-naphthalenedicarboxylate) (PEN). These polyesters typically have good heat resistance and high glass transition temperatures.
For those applications in which extrusion and molding temperatures must be maintained below about 240.degree. C., these highly crystallizable polyesters are not used because their melting points are too high. In such cases, amorphous or slowly crystallizable copolyesters are used since these materials may be processed at moderate temperatures.
For applications of packaging of static sensitive electronic components such as disk drive heads and integrated circuits, materials that are conductive or static dissipative and processible at moderate temperatures are required. The optimum fitness-for-use criteria for this market include electrostatic dissipation properties, dimensional stability, washability, thermoformability, acceptable slitting characteristics, peelable seal characteristics to cover tapes, and low migration levels of condensable materials from the package to the packaged components. An additional optimum criterion is transparency so that the component may be seen through the package. Since inherently dissipative polymers typically do not have the mechanical properties needed to form useful articles for packaging, they need to be blended with other polymers, such as amorphous or semi-crystalline copolyesters, to achieve useful properties.
WO 91/10237 discloses static dissipative compositions containing a non-conductive matrix polymer and at least two additives. In one of the examples a copolyester of poly(ethylene terephthalate) containing cyclohexanedimethanol and ethylene glycol is combined with polyaniline.
U.S. Pat. Nos. 4,719,263, 4,931,506, 5,101,139 and 5,237,009, all assigned to The B.F. Goodrich Company, disclose the use of ethylene oxide copolymers and epihalohydrin copolymers in imparting electrostatic dissipating properties to various polymers including thermoplastic polyesters.
U.S. Pat. Nos. 5,159,053 and 5,342,889 disclose electrostatic dissipating polymers based on polyurethanes derived from polyethylene glycol compositions including the commercial product known as Stat-Rite, available from The B.F. Goodrich Company. The polyurethanes may be blended with other base polymers as an electrostatic dissipative (EDS) agent.
Other sources of prior art on EDS materials include J. A. Harding et al, "Static Dissipative Polymeric Antistatic Modified Thermoplastics," SPE ANTEC Technical Papers, 36, 409 (1990) and Bing-Lin Lee, "Electrically Conductive Polymer Composites and Blends," Polymer Engineering and Science, 32, 36 (1992). The latter describes certain properties of an 80/20 PETG/polyether copolymer.